For industrial applications, it is sometimes desired to accurately position items in assembly or manufacturing processes, such as in a packaging, tensioning, positioning, stacking, guiding, pick-and-place or other industrial automation applications. Many times, pneumatic actuators are used to provide the motive force for such applications. In simple operations, certain simple pneumatic actuators are utilized. The simplest types are 2-position pneumatic actuators only capable of stopping at the end positions, i.e., all the way to one end or all the way to the other end of the stroke. Although cost effective, they are only useful in a very limited set of automation applications.
More sophisticated pneumatic actuators, such as the TOM THUMB.RTM. 3-position pneumatic actuator sold by PHD, Inc. of Fort Wayne, Ind., includes the ability to stop at an intermediate or middle position. Although more flexible than 2-position actuators, these 3-position actuators are still very inflexible, in that, once designed, the intermediate position is largely unchangeable.
In the next level of sophistication, actuators are available which can stop at any intermediate position. For example, SMC Corporation of Tokyo, Japan manufactures a rodless pneumatic cylinder with an internal brake and positioning scale (e.g. model ML2B). This system includes a piston moveable within a housing and integral position sensor and a friction brake. The position sensor provides a position signal to the controller. By comparing the instantaneous position with inputted desired position data, the brake is actuated via air pressure to move a brake shoe into contact with a brake plate, thereby stopping the piston at the predetermined intermediate point. The system includes the ability to learn the distance from application of the brake to the actual stopping point, and makes adjustments to improve the accuracy for at the next commanded stop.
Adding the ability to stop at an intermediate position in such pneumatic systems is very desirable, however, such friction braking tends to add significant mechanical and pneumatic complexity and additional expense to the system. Moreover, such systems can only be full on or full off at any position along the actuator stroke, thus, by their very nature they are inflexible. Moreover, such systems tend have good accuracy only at low speeds.
Robohand, Inc. of Monroe, Conn., manufactures pneumatic position control systems under the tradename POSITIONEX.TM.. These systems include a pneumatic servo-actuator having a piston moveable in, and subdividing, a cylinder into a first and second chamber. They also include an output shaft interconnected to the piston, a position transducer providing a signal of a position of the output shaft and a servo-valve controlled by a control system to apply the appropriate pressure to position the output shaft at the appropriate predetermined position. Problematically, such systems tend to overshoot and hunt (oscillate about) the desired stopping position. Moreover, the servo-valves tend to be very complex and expensive.
Accordingly there has been a long felt, and unmet need for a cost effective pneumatic actuator and positioning system which is capable of accurately stopping at any point along its stroke.